Inductively coupled plasma-mass spectrometry (ICP-MS) is often utilized for elemental analysis of a sample, such as to measure the concentration of trace metals in the sample. An ICP-MS system includes a plasma-based ion source to generate plasma to break molecules of the sample down to atoms and then ionize the atoms in preparation for the elemental analysis. In a typical operation, a liquid sample is nebulized, i.e., converted to an aerosol (a fine spray or mist), by a nebulizer (typically of the pneumatic assisted type) and the aerosolized sample is directed into a plasma plume generated by a plasma source. The plasma source often is configured as a flow-through plasma torch having two or more concentric tubes. Typically, a plasma-forming gas such as argon flows through an outer tube of the torch and is energized into a plasma by an appropriate energy source (typically a radio frequency (RF) powered load coil). The aerosolized sample flows through a coaxial central tube (or capillary) of the torch and is emitted into the as-generated plasma. Exposure to plasma breaks the sample molecules down to atoms, or alternatively partially breaks the sample molecules into molecular fragments, and ionizes the atoms or molecular fragments.
The resulting analyte ions, which are typically positively charged, are extracted from the plasma source and directed as an ion beam into a mass analyzer. The mass analyzer applies a time-varying electrical field, or a combination of electrical and magnetic fields, to spectrally resolve ions of differing masses on the basis of their mass-to-charge (m/z) ratios, enabling an ion detector to then count each type of ion of a given m/z ratio arriving at the ion detector from the mass analyzer. Alternatively the mass analyzer may be a time of flight (TOF) analyzer, which measures the times of flight of ions drifting through a flight tube, from which m/z ratios may then be derived. The ICP-MS system then presents the data so acquired as a spectrum of mass (m/z ratio) peaks. The intensity of each peak is indicative of the concentration (abundance) of the corresponding element of the sample.
In addition to analyte ions for which analysis is sought, the plasma produces background (non-analyte) ions. Certain types of non-analyte ions, referred to as interfering ions, can interfere with the analysis of certain types of analytes. The interfering ions may be produced from the plasma-forming gas (e.g., argon), matrix components of the sample, solvents/acids included in the sample, or air (oxygen and nitrogen) entrained into the system. For example, the interfering ions may be isobaric interferents that have the same nominal mass as an analyte ion. The detection of such interfering ions along with the detection of certain analyte ions leads to spectral overlap in the analytical data, thereby reducing the quality of the analysis. Examples of interfering ions include polyatomic ions such as argon oxide, 40Ar16O+, which interferes with the iron isotope 56Fe+ because both ions appear at m/z=56 in mass spectra, and argon 40Ar+, which interferes with the calcium isotope 40Ca+ because both ions appear at m/z=40.
Known approaches for addressing the problem of spectral interference and improving the performance of an ICP-MS system have involved improvements in matrix separation, the use of cool plasma technology, and the use of mathematical correction equations in the processing of the analytical data. These approaches have known limitations. To further address the problem, it is also known to provide a collision/reaction cell in the ICP-MS system between the ion source and the mass analyzer. The cell includes an ion guide that focuses the ion beam along the central axis of the cell. The cell is filled with either a collision gas or a reactive gas. The use of a collision gas (e.g., helium, He) relies on kinetic energy discrimination (KED) by which polyatomic ion interference can be suppressed. Both the analyte ions and the polyatomic interfering ions in the cell undergo multiple collisions with the collision gas molecules, and lose kinetic energy (KE) and thus are decelerated as a result. However, because the polyatomic ions have larger cross-sections than the analyte ions, the polyatomic interfering ions undergo a greater number of collisions and thus lose more kinetic energy than the analyte ions. KED can therefore be utilized to separate the analyte ions from the polyatomic interfering ions, as appreciated by persons skilled in the art.
Theoretically, when carrying out the ion-molecule collision process, the degree of the interference reduction increases with the average number of collisions the ions experience. For example, when Fe+ and ArO+ experience 20 and 40 collisions on average with helium, respectively, the ion intensity ratio Fe+/ArO+ is higher than when the two ions experience 10 and 20 collisions on average. A higher collision gas (e.g., He) density in the cell (flow rate) is therefore preferable. But this is only true as long as the analyte ions (e.g., Fe+) maintain sufficient post-collision KE to surmount the DC potential barrier. In other words, the collision/reaction cell performance is limited by the upper limit of collision gas density (flow rate), which corresponds to the thermalization (and consequently stalling) of the analyte ions.
Therefore, there is a need for an improved collision/reaction cell and method for performing ion-molecule collisions to address the problem of interferences.